Six fire engines and around 40 firefighters tackled a fire at a derelict building on Elmore Drive in Harrow.
The ground and first floors were destroyed and part of a single storey side annexe was damaged by the fire. There are no reports of any injuries.
Two of the Brigade's 32-metre turntable ladders were used as water towers to help fight the fire from above.
Control Officers took the first of 12 calls at 2253 and mobilised crews from Stanmore, Harrow, Mill Hill and surrounding fire stations to the scene. The fire was under control by 0019.
The cause of the fire has been recorded as undetermined.